We develop a model for TCP that accounts for both sublinearity and limitation of window increase. Sublinear window growth is observed when the round-trip time of the connection increases with the window size. The limitation is due to the window advertised by the receiver. First, we derive the required conditions for the stability of the model. Then, we write the Kolmogorov equation under Markovian assumptions. The model is solved 1analytic- ally for some particular cases. A good match between the throughput predicted by the model and the throughput measured on real TCP connections is reported.